1. Field of the Invention
The present invention relates to a photovoltaic device. More specifically, the present invention relates to a photovoltaic device using amorphous silicon as a semiconductor material.
2. Description of the Prior Art
A photovoltaic device such as a solar battery, photosensor and the like is capable of directly converting solar radiation and the like into electrical energy. However, such photovoltaic devices involve a great problem in that the electricity generating cost is extremely high as compared with that of other electrical energy generating means. Major reasons for the above are that the utilization efficiency of the semiconductor material constituting a main portion of the device is low and the cost required for manufacturing such a semiconductor material is very high.
However, of late a photovoltaic device using a more inexpensive semiconductor material which solves the above described problems has been proposed. A typical photovoltaic device employing such material is disclosed in U.S. Pat. No. 4,064,521, issued Dec. 20, 1977 to David Emil Carlson and entitled "SEMICONDUCTOR DEVICE HAVING A BODY OF AMORPHOUS SILICON". Amorphous silicon used in such a photovoltaic device can be formed at low cost and on a mass production basis of a glow discharge in an atmosphere of a silicon compound such as silane, silicon fluoride or the like. Such amorphous silicon has an average local state density in the width of forbidden band as low as 10.sup.17 cm.sup.-3 or less, with the possibility of control of a P type or N type impurity, as in the case of crystal line silicon.
FIG. 1 shows one example of a typical conventional photovoltaic device employing amorphous silicon which constitutes the background of the invention. Referring to FIG. 1, an amorphous silicon device comprises a P type layer 3, an intrinsic layer 4 and an N.sup.+ layer 5. A transparent electrode 2 and an ohmic contact electrode 6 are formed on opposite surfaces of the amorphous silicon layer and such composite is formed on a glass substrate 1. The N.sup.+ amorphous silicon layer 5 is formed as a highly impurity doped layer so as to serve as an ohmic contact to the electrode 6.
When a light beam impinges upon the amorphous silicon layer through the glass substrate 1 and the transparent electrode 2 in such a photovoltaic device as shown in FIG. 1, electrons and/or holes of a free state are generated mainly in the intrinsic layer 4. These are drawn by the PIN (I=Intrinsic) junction electric field formed in the amorphous silicon layer so as to be transported to and collected at the transparent electrode 2 and the ohmic electrode 6 serving as collecting means. Accordingly, a photovoltaic current of the magnitude associated with the intensity of the incidental light beam is withdrawn between the transparent electrode 2 and the ohmic electrode 6.
The FIG. 1 photovoltaic device produces a voltage of approximately 0.8 V under no load (Voc) conditions (and of 0.3 to 0.5 V as an operational voltage (Vop)) and as such cannot be used as a voltage source for equipment requiring a higher voltage.
On the other hand, it has been proposed that a plurality of small voltage generating devices be formed on a common substarate, thereby providing the necessary larger voltage. Such an apparatus is disclosed in, for example, Japanese Utility Model Publication Gazette No. 26064/1976 published for opposition July 20, 1976. However, the above referenced Japanese Utility Model Publication Gazette No. 26064/1976 shows an apparatus employing devices using selenium as a semiconductor material and fails to show a device employing amorphous silicon which is of most interest to the present invention.
Consequently, an implementation of a photovoltaic device such as disclosed in the above reference U.S. Pat. No. 4,064,521 in such an arrangement as disclosed in the above referenced Japanese Utility Model Publication Gazette No. 26064/1976 through mere aggregation thereof does not provide a device having excellent characteristics and mass productivity. More specifically, a composite selenium photocell disclosed in the above referenced Japanese Utility Model Publication Gazette No. 26064/1976 has a structure wherein a plurality of selenium photocells are formed on a single substrate such that semiconductor layers constituting the respective photocells are isolated from each other. Such formation of isolated semiconductor layers, however, results in a poor mass productivity because of the requirement of registration and the like. Furthermore, simple substitution of amorphous silicon disclosed in the above referenced U.S. Pat. No. 4,064,521 for the material of such semiconductor layers in the structure disclosed in the above referenced Japanese Utility Model Publication Gazette No. 26064/1976 further entails the following problem. More specifically, although in the case of the device disclosed in the above referenced Japanese Utility Mode Publication Gazette No. 26064/1976 formation of an oxide such as cadmium oxide occurs as the last step, the step of forming such oxide would damage the amorphous silicon layer. For example, since the P type layer of the amorphous silicon layer is as thin as approximately 50 A to 100 A, such oxide forming process would deteriorate the amorphous silicon layer and in the worst case the P type layer could disappear. Accordingly, conversion efficiency becomes lower in such damaged portions and hence a photovoltaic device of good efficiency is not provided. Furthermore, an approach could be thought of wherein a semitransparent metallic film is formed as a light transmissive film in the above referenced Japanese Utility Model Publication Gazette 26064/1976. However, in such a case, it is extremely difficult to form a metallic film of good light transmission and good electric conductivity. More specifically, in order to be more light transmissive, the light transmissive film needs to be thinner; however, it is extremely difficult to control the film thickness of such a film and to attain good reproducibility. Accordingly, even in such an approach, the mass productivity would be very poor as compared with that of the present invention.